Complex-transition metal oxides comprise a large group of materials that show an extremely diverse and useful range of characteristics. Examples of but a few technologically useful complex-transition metal oxide materials include ferrimagnetic materials having the spinel, garnet, or hexaferrite crystallographic structures, ferroelectric materials having the perovskite crystallographic structure, and perovskites that display superconducting behavior at liquid nitrogen temperatures. All of these examples are of note because they are both of great technological interest as well as manifestations for the presence of long range cooperative order in the materials. Because of their potential impact on modern electronics, photonics, and microelectromechanical systems (MEMS) applications they are being intensively researched. However, their novel properties derive from complex crystallographic structures that heretofore have been difficult to integrate with the standard processing techniques used by the electronics, photonics and MEMS communities.
The basis of present and future trends in electronics, photonics, and MEMS involves the extension of large-scale batch fabrication of devices on semiconductor or dielectric wafer materials. Clearly, the effects of such large scale integration has yielded ubiquitous, useful devices. Unfortunately, it has thus far been difficult to integrate complex-transition metal oxide devices with current processes, where here integration is denoted as being able to fabricate both complex-transition metal oxide devices and semiconductor-based devices on the same substrate material using methods compatible with semiconductor batch processing techniques. There are many reasons why complex-transition metal oxide materials have not been integrated with semiconductor materials using standard processing techniques, including the large thermal expansion mismatches between materials, and the need to grow the complex-transition metal oxides in oxygen atmospheres at temperatures much higher than those allowable to avoid diffusion or degradation of either the wafer material or metallization layers.
It should be recognized here that the complex-transition metal oxides described in this document do not incorporate the simple oxide materials, such as SiO.sub.2, Al.sub.2 O.sub.3 or the many glasses that are commonly used in semiconductor processing for insulating or passivating layers. Nor does it refer to simple transition metal oxide materials, such as the conductors ZnO and RuO.sub.2, which can be deposited on wafers using temperatures and processes compatible with standard semiconductor fabrication techniques. Here, the complex-transition metal oxides refer to materials having large chemical formula units and complex unit cells, and are possessed of long-ranged cooperative phenomenon such as ferrimagnetic, ferroelectric, or superconducting properties. These materials are of value for integrated devices because of the unique properties that arise from the long-ranged cooperative phenomenon, for example nonreciprocal wave propagation in ferrimagnets, very high controllable polarizabilities in ferroelectrics, and very high conductivities in superconductors. In turn, the presence of long-range cooperative phenomenon, and the quality of the resulting effects, requires the oxide film to have high structural and chemical ordering at the atomic level. Unfortunately, the process requirements needed for growing high quality complex-transition metal oxide films typically conflicts with the requirements used for producing integrated devices.
Moreover, the problems that beset integrating complex-transition metal oxide films with standard semiconductor fabrication techniques also hold for some intermetallic alloy systems that also have large chemical formulas and complex unit cells, and possess long ranged cooperative phenomenon. It would be very technologically useful to integrate intermetallics such as the rare earth-containing ternary or quarternary borides, which have very high magnetic energy products and hence are very good permanent magnets, routinely into electronics or MEMS applications.
One example of the difficulty of integrating complex-transition metal oxides and semiconductors is shown by the requirements for producing integrated ferrimagnetic garnet devices, such as circulators or phase-shifters, on semiconductor wafers for monolithic microwave integrated circuit (MMIC) applications. The performance advantages of fabricating devices such as integrated garnet/silicon circulators has been asserted for almost two decades. In using the present direct deposition method to meet this goal, high quality garnet films must be deposited at temperatures compatible with the other materials present on the wafer, i.e. below 500.degree. C. Moreover, the garnet films must be up to 200 micrometers thick, several hundred-times thicker than most film depositions used in fabricating MMIC devices. Unfortunately, garnet films deposited at temperatures of 500.degree. C. are structurally amorphous, and show no useful ferrimagnetic properties. It is only by growing these films at temperatures above 700.degree. C., or by post-annealing the wafers at these high temperatures, that the garnet films become polycrystalline and thus possess useful ferrimagnetic properties. This procedure can still degrade the properties of other materials on the wafer unless great care is taken in the ordering of the total wafer fabrication process. Thus, only recently has a direct film deposition process been developed that overcomes this substantial materials problem, and others, to yield high quality integrated polycrystalline garnet/silicon circulators.
Despite the hard-won successes of the direct film deposition technique, the resulting complex-transition metal oxide films do not necessarily possess optimal material properties since they are polycrystalline, and not structurally highly-oriented or single-crystal. For most applications involving complex transition-metal oxide films, devices made with polycrystalline films have reduced performance or increased losses compared to single-crystal films. For example, devices made from polycrystalline ferrimagnetic films will have higher magnetic loss compared to the same device made from a single-crystal film, while any imperfections in a high-temperature superconducting film will reduce its current carrying capacity. Thus, in general the highest performance integrated complex-transition metal oxide devices will be produced using single-crystal oxide films.